The present invention relates to a method and apparatus for changing the direction of motion of a pallet moving on wheels at the end of a transport conveyor or the like.
Like escalators, transport conveyors are transport devices designed to move people or goods. They differ from escalators, e.g., in that they are often operated in a substantially horizontal position or in a position somewhat inclined relative to their direction of motion, typically by 1-15°, so that successive steps, i.e., pallets form a substantially even and linear track instead of stair-like steps as in the case of escalators. Instead of successive steps, transport conveyors may also have a continuous belt, in which case the transport conveyor resembles a belt conveyor. Transport conveyors are also called moving walkways and autowalks.
In prior-art transport conveyors, the structure is typically embedded in the ground or in the floor of the building. In this case, a pit about one meter deep and several meters long is provided at either end of the transport conveyor for the drive machinery of the transport conveyor structure and for the mechanism turning the pallets. Correspondingly, the depth of the recess needed in the middle part of the transport conveyor is about half a meter. One drawback with these types of transport conveyor constructions is that they require heavy and fixed structures in the floor of the surrounding area, which must be taken into account at the time of designing the buildings. Another drawback is that transferring fixed structures like these from one place to another (e.g., in response to changing traffic needs) is completely impossible.
In prior-art solutions, the pallet track typically consists of pallets that are about 15-40 cm long, corresponding to the steps of escalators. The pallets are usually chained together by means of a chain, e.g., a cogged belt, and the whole chain is driven by a machine having a sprocket. The upper pallets in the chain move on rollers supported by a special track. At the end of the pallet track, the pallets turn upside down around a wheel of a large diameter and return back to the beginning of the track by the lower side of the track. At the beginning of the track, the pallets again turn around to their normal traveling position and move further on the upper side of the track towards the end of the track.
One of the problems with this type of prior art solution is the turning-over motion of the pallets at the ends of the track, because the height of the turning space has to be at least equal to the length of the pallet. In practice, however, the height is typically about 1.5-2 times the length of the pallet in order to provide adequately smooth operation.
In British Patent Document No. GB2299316, FIG. 1/3 shows a structure of the above-described type as an example of the prior art. In addition, this document discloses a transport conveyor construction wherein the pallets are guided by fixed tracks at their turning points while being continuously held in a horizontal position. Thus, the pallet is not turned upside down at the turning point. Control of the pallets is maintained by means of a curved track for the leading wheels (in the direction of motion of the pallet) while the trailing wheels are substantially free. However, this solution involves the problem that the track has so-called dead centers at its end points where the position of the pallet is not precisely controlled. As a consequence, the pallet may get jammed at the dead center, which can cause the entire apparatus to stop as a result of the operation disturbance, and may even be damaged. The risk of jamming is increased by the running clearances, non-ideal track properties, and wear of the mechanism.